1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
In this aÂ­rticle, we will talk about how the eye focuses, how a lens works, how to read a prescription, and finally, how the lens is made, including the steps involved in grinding and shaping plastic lens blanks to fit an individual's prescription and frame. Let's start with some vision basics.